This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Motility is an important component of the pathogenesis of bacteria and the bacterial flagellar motor is considered to be the most proficient biological machine for this purpose. The influx of protons through the motor produces either counterclockwise or clockwise flagellar rotation, resulting in translational motion or 'tumbling', respectively. Despite decades of research, the precise mechanism of flagellar motor rotation has remained a mystery. The purpose of proposal is to provide the most detailed structural insight into the flagellar motor using high-throughput cryo-electron tomography (cryo-ET), and to apply this information to a testable model of flagellar rotation and motor reversal. This work will in turn provide the molecular basis on the mechanism of motility in both pathogenic and nonpathogenic bacteria. Borrelia burgdorferi is ideally suited for this type of analysis because of 1) its small diameter (~0.2 [unreadable]m), which is necessary for high resolution cryo-ET;and 2) the availability of ~3,600 transposon mutants.